This invention relates generally to neutral beam injection systems for use in controlled fusion devices such as the tokamaks, magnetic mirror systems, bumpy tori and the like, and more particularly, this invention relates to improvements in neutral beam injectors with direct ion energy recovery.
In the field of controlled fusion, or controlled thermonuclear reaction, a high temperature plasma is formed of light isotope ions whose nuclei are fusionable and which are contained within a magnetic field confinement, or containment zone, in an evacuated region. Such light isotopic species may generally comprise one or more materials, such as hydrogen, deuterium, tritium, helium 3 etc., whose nuclei undergo fusion reactions under appropriate conditions of confinement time, density, and temperature. These conditions may be brought about or supplemented by the injection of properly accelerated neutral particle beams of one or more of the appropriate species on the proper trajectory with respect to the magnetic field. A portion of the energetic neutral particles are ionized by collision with neutral or charged particles and are accordingly trapped by the magnetic field to form a high temperature plasma. The injected particles must be neutral in order to penetrate the very strong magnetic-field-contained plasma.
Since the neutral particles cannot be directy accelerated to high velocity, i.e., high kinetic energies, they are produced in an indirect manner from an ion source. The neutral beam may be produced by accelerating either positively or negatively charged ion, of one or more of the above species, emerging from an ion source and then passing the charged particles through a gas-cell neutralizer wherein they interact with neutral atoms and molecules of the neutralizer gas at a specified pressure through charge exchange. A portion of the ions are neutralized and emerge from the neutralizer as high energy neutral particles along with the accelerated beam passing therethrough. The beams are generated and manipulated in a vacuum chamber whose pressure is maintained at the selected pressure level by a cryopumping system.
Since the beam emerging from the neutralizer also contains electrons and unneutralized ions, some means must be provided to separate the neutral particles from the electrons aned ions to obtain the desired neutral beam for injection into the magnetically confined plasma. This is accomplished by either electrostatic or magnetic field blocking of the electrons, and diversion or bending of the positively charged ions from the primary beam path direction to allow the neutral beam to continue along the accelerated beam path. Depending upon the species and energy of the initially ionized particles of the beam, the neutral particle beam emerging from the neutralizer contains a large portion of high-energy unneutralized ions. Present ion sources operating at energies of about 40 kilo-electron volts (keV) per nucleon at 60 amps ion current provide about a 60% conversion efficiency in the neutralizer cell. As future ion sources ae developed toward energies of about 100 keV, or higher, per nucleon, at comparable current levels, the conversion efficiency drops to about 13% for hydrogen neutrals using H.sub.2 gas and about 52% for deuterium nuetrals using D.sub.2 gas, which represents an intolerable energy loss.
Therefore, in order to produce neutral beams for fusion plasma heating efficiently, the energy contained in the unneutralized fraction of the beam must be recovered. In order to recover the kinetic energy of the charged ion component of the beam emerging from the neutralizer cell, in the form of usable electric energy, the electrons present in the beam must be blocked and the beam ions diverted from the neutral beamline, decelerated and collected. The electrons must be blocked from entering the ion collector area since they would be accelerated into the ion collector thereby producing an energy loss which may be equal to or greater than the recovered ion energy.
In the process of developing direct energy recovery in neutral beam injectors, various means have been devised or suggested, which may be generally divided into two groups, depending upon the ion-deflection method used. They are either electrostatic or magnetic ion-deflection methods.
An electrostatic deflection system is described in "Proceedings of 7th Symposium on Engineering Problems of Fusion Research," 1978, by W. L. Barr et al, Vol. 1, page 308. This paper discloses an electrostatic system developed at Lawrence Livermore Laboratory, Livermore, Calif., in which the neutralizer cell wall is held at ground potential, the ion beam collector is biased highly positive (approximately 100 kV) and the electrons emerging from the cell are repelled by a negative voltage (approximately 20 kV) applied to the electrodes which closely encompass the beam. One negative electrode is placed between the neutralizer cell exit and a generally funnel-shaped ion collector which also emcompasses the beam. The other negative electrode is placed at the exit of the collector. The ion collector acts to decelerate and collect the ions diverging radially from the beam. The negative electrodes in this system must be biased sufficiently negative to drive the beam potential negative even on the axis in the presence of the positive-ion space charge and the nearby positive-ion collector. There are inherent problems with this system which include severe gas pressure requirements for efficient direct conversion, increased beamline length in order to establish the retarding electric field which consequently reduces the neutral power transmission efficiency and the need to hold a high positive potential on the ion collector in the presence of spatial and time-varying magnetic fields. The most critical gas-pressure requirement placed on this direct conversion system is imposed by the power load resulting from the acceleration and collection of the slow ions and electrons produced by ionization and charge exchange of the background gas. The resulting emission of secondary electrons at negative high voltage and the subsequent power drain must also be considered.
Other electrostatic electron-blocking and ion-deflection systems utilizing electrostatic grids which can intercept the beam are discussed by P. Raimbault in EUR-CEA-FE-823, 1976. One specific system outlined in this reference employs a cylindrical grid arrangement which surrounds the beam exiting the neutralizer which is biased negative with respect to the neutralizer to suppress the electrons. The ion collection method of this system has one advantage and one disadvantage compared to the Barr system mentioned above. The single advantage is that the ion collector is at ground potential. However, in addition to the other disadvantages to the Barr system, the Raimbault system also suffers from direct interception of the ion beam on the high negative potential, cylindrical grid. Not only is the ion energy lost, but secondary electrons ejected from the grid by the ion impingement constitute an additional power loss. In the proposed Raimbault system, the ion source is operated at near ground potential, and the ions are accelerated by operating the neutralizer at a high negative potential. The positive potential, at which the ion source is held above ground potential, is necessary to ensure that the unneutralized ions are able to reach the ion collector plate.
By operating the neutralizer at a negative potential to accelerate the ions from the source through the neutralizer makes it possible to recover the energy of the ions by deceleration to ground potential and eliminates the problems associated with a high positive potential deceleration voltage on an ion collector for recovery of their kinetic energy directly.
Further, as pointed out above, it has been suggested in the art to employ magnetic means for deflecting the ions from the accelerated beam, and it has been further suggested to employ magnetic suppression, or blocking, of the electrons from the beam emerging from the neutralizer tube. It has been recognized in the art that magnetic suppression would be advantageous in that the magnetic field can penetrate beams that are too thick and too dense for electrostatic suppression to work. However, in the prior-art experiments employing magnetic suppression, electrostatic fields also present in the system from positive potential deceleration ion collectors, have produced unnecessarily long beamlines and/or complicated electron motions which produce long-lived electrons in the system. Some of these electrons cause unwanted power drain or tend either to re-ionize the neutral beam exiting the neutralizer cell or de-ionize the positive ions directed to the energy recovery ion collectors.
A U.S. Pat. No. 4,349,505 of common assignee with the present invention, field July 1, 1980, by William L. Stirling for "Neutral Beamline With Ion Energy Recovery Based On Magnetic Blocking of Electrons" discloses a system employing magnetic blocking of the electrons and electron collection at the neutralizer exit. The neutralizer is operated at a high negative acceleration potential and the emerging beam experiences a strong electric field due to the surrounding ground potential structure which is transverse to the magnetic field applied across the beam at the neutralizer exit. Any electrons present in the beam from leakage or from secondary emission are quickly moved out of the beam due to E.times.B field drift and directed into a slightly positive biased, electron collector. However, molecular ions of the selected species extracted from the ion source along with the atomic ions are dissociated into atomic particles as they pass through the near-equilibrium gas cell. These particles have kinetic energies of fractional values with respect to the original full acceleration energy (E) of the atomic ions accelerated through the neutralizer and cannot reach the ground-potential surfaces on which the full energy (E) ions are collected. These fractional energy ions are deflected along paths of substantially smaller radius and tend to impinge upon the outer walls of the neutralizer cell producing undesirable secondary electrons outside of the electron collector region. These electrons are then accelerated to the surrouding ground potential surfaces intended for the collection of the full energy ion's charge and detract from the energy recovery efficiency of the full energy ions.
Therefore, it will be appreciated that there is a need for improvements in neutral beamline systems with ion energy recovery based on the advantages of magnetic blocking of electrons and beam ion deflection which deals with the fractional energy ions exiting the neutralizer cell to prevent their interferring with the recovery of the energy from the full energy ions exiting the cell.